24 The Botanical Survey of Nebraska 



HUMIDITY AND TEMPERATURE 



In the first paper in this series Weaver and Thiel have shown 

 how relatively low humidities, high temperatures, and high wind 

 velocities contribute to maintaining the higher evaporating power 

 of the air in the prairie as compared to that in the thicket and 

 woodland station. 



The general trend of evaporation in the three stations was con- 

 siderably higher during, the season of 191 7 than in 191 6, as has 

 already been noted. This difference is significant as to the rela- 

 tive dryness of the air for the several stations and for the two 

 seasons because in 191 7 the atmometers were run at a lower level 

 than in 1916. The porous, evaporating surfaces were more or 

 less protected by the surrounding vegetation from the full effect 

 of the wind. This was particularly true, of course, for the 

 prairie station. 



An examination of our humidity-temperature records reveals 

 the fact that the minimum humidities recorded in the prairie are 

 almost without exception lower than those in the shrub or wood- 

 land stations. The minimum humidity in the prairie and shrub 

 communities approached each other rather closely on a number 

 of days during the early part of the summer, but these were infre- 

 quent and for only relatively brief periods. These facts are 

 shown on the graphs for the week of April 25, and they are in 

 general true only for that period of spring and early summer 

 before the plants of the shrub habitat have developed their full 

 foliage density. Other series of records show considerable dif- 

 ferences in the minimum humidities of the three stations, and 

 these are to be taken as illustrative of the greater portion of the 

 growing season. But naturally the relatively low minimum 

 humidity of the shrub community during the physiologically im- 

 portant fore-summer may be very significant as contributing to 

 the difficulties of ecesis therein. A period of low relative humid- 

 ity of late May, 191 7, was reflected in a striking manner by a 

 great increase in evaporation in all of the stations. 

 ■ For a very short period during the driest time of the day the 

 humidity in the shrub station was occasionally slightly lower than 

 that for the same day in the prairie, but as in 1916, such minima 



30 



